Low viscosity kraft fiber having an enhanced carboxyl content and methods of making and using the same

ABSTRACT

A pulp fiber with an enhanced carboxyl content resulting in improved antimicrobial, anti-yellowing and absorptive properties is described. Methods for making the kraft pulp fiber and products made from it are also described.

This is a divisional of U.S. application Ser. No. 14/206,744, filed Mar.12, 2014, now U.S. Pat. No. 9,951,470, which claims the benefit of U.S.Provisional Application No. 61/787,146, filed Mar. 15, 2013, each ofwhich is incorporated herein by reference.

This disclosure relates to modified acid-treated kraft fiber having anenhanced carboxyl content resulting in, among other things, better waterabsorptive capacity, improved wet and dry strength, improvedanti-yellowing characteristics and a lower pH. More particularly, thisdisclosure relates to a kraft fiber, e.g., softwood fiber, that has beentreated to result in a unique set of characteristics, improving itsperformance over untreated and other treated fiber.

This disclosure further relates to chemically modified cellulose fiberderived from bleached softwood that has an enhanced carboxyl content,making it suitable for use as a chemical cellulose feedstock in theproduction of cellulose derivatives including cellulose ethers, esters,and viscose, as fluff pulp in absorbent products, and in other consumerproduct applications. As used herein, “degree of polymerization” may beabbreviated “DR” “Ultra low degree of polymerization” may be abbreviated“ULDP.”

This disclosure also relates to methods for producing the improved fiberdescribed. The fiber, described, is subjected to digestion and oxygendelignification, followed by bleaching. The fiber is also subject to acatalytic oxidation treatment and a carboxylating acid treatment. Insome embodiments, the fiber is oxidized with a combination of hydrogenperoxide and iron or copper and then further bleached to provide a fiberwith appropriate brightness characteristics, for example brightnesscomparable to standard bleached fiber. Further, at least one process isdisclosed that can provide the improved beneficial characteristicsmentioned above. The fiber can be oxidized in a kraft process, such as akraft bleaching process and then subjected to a carboxylating acidtreatment. Still a further embodiment relates to process includingfive-stage bleaching comprising a sequence of D₀E1D1E2D2, where one ormore of the E1 or E2 stages comprises the catalytic oxidation treatmentand then subjecting the fiber to carboxylating acid treatment.

Finally, this disclosure relates to products produced using the improvedmodified acid-treated kraft fiber as described.

Cellulose fiber and derivatives are widely used in paper, absorbentproducts, food or food-related applications, pharmaceuticals, and inindustrial applications. The main sources of cellulose fiber are woodpulp and cotton. The cellulose source and the cellulose processingconditions generally dictate the cellulose fiber characteristics, andtherefore, the fiber's applicability for certain end uses. A need existsfor cellulose fiber that is relatively inexpensive to process, yet ishighly versatile, enabling its use in a variety of applications.

Kraft fiber, produced by a chemical kraft pulping method, provides aninexpensive source of cellulose fiber that generally provides finalproducts with good brightness and strength characteristics. As such, itis widely used in paper applications. However, standard kraft fiber haslimited applicability in downstream applications, such as cellulosederivative production, due to the chemical structure of the celluloseresulting from standard kraft pulping and bleaching. In general,standard kraft fiber contains too much residual hemi-cellulose and othernaturally occurring materials that may interfere with the subsequentphysical and/or chemical modification of the fiber. Moreover, standardkraft fiber has limited chemical functionality, and is generally rigidand not highly compressible.

In the standard kraft process a chemical reagent referred to as “whiteliquor” is combined with wood chips in a digester to carry outdelignification. Delignification refers to the process whereby ligninbound to the cellulose fiber is removed due to its high solubility inhot alkaline solution. This process is often referred to as “cooking.”Typically, the white liquor is an alkaline aqueous solution of sodiumhydroxide (NaOH) and sodium sulfide (Na₂S). Depending upon the woodspecies used and the desired end product, white liquor is added to thewood chips in sufficient quantity to provide a desired total alkalicharge based on the dried weight of the wood.

Generally, the temperature of the wood/liquor mixture in the digester ismaintained at about 145° C. to 170° C. for a total reaction time ofabout 1-3 hours. When digestion is complete, the resulting kraft woodpulp is separated from the spent liquor (black liquor) which includesthe used chemicals and dissolved lignin. Conventionally, the blackliquor is burnt in a kraft recovery process to recover the sodium andsulphur chemicals for reuse.

At this stage, the kraft pulp exhibits a characteristic brownish colordue to lignin residues that remain on the cellulose fiber. Followingdigestion and washing, the fiber is often bleached to remove additionallignin and whiten and brighten the fiber. Because bleaching chemicalsare much more expensive than cooking chemicals, typically, as muchlignin as possible is removed during the cooking process. However, it isunderstood that these processes need to be balanced because removing toomuch lignin can increase cellulose degradation. The typical Kappa number(the measure used to determine the amount of residual lignin in pulp) ofsoftwood after cooking and prior to bleaching is in the range of 28 to32.

Following digestion and washing, the fiber is generally bleached inmulti-stage sequences, which traditionally comprise strongly acidic andstrongly alkaline bleaching steps, including at least one alkaline stepat or near the end of the bleaching sequence. Bleaching of wood pulp isgenerally conducted with the aim of selectively increasing the whitenessor brightness of the pulp, typically by removing lignin and otherimpurities, without negatively affecting physical properties. Bleachingof chemical pulps, such as kraft pulps, generally requires severaldifferent bleaching stages to achieve a desired brightness with goodselectivity. Typically, a bleaching sequence employs stages conducted atalternating pH ranges. This alternation aids in the removal ofimpurities generated in the bleaching sequence, for example, bysolubilizing the products of lignin breakdown. Thus, in general, it isexpected that using a series of acidic stages in a bleaching sequence,such as three acidic stages in sequence, would not provide the samebrightness as alternating acidic/alkaline stages, such asacidic-alkaline-acidic. For instance, a typical DEDED sequence producesa brighter product than a DEDAD sequence (where A refers to an acidtreatment).

Cellulose exists generally as a polymer chain comprising hundreds totens of thousands of glucose units. Cellulose may be oxidized to modifyits functionality. Various methods of oxidizing cellulose are known. Incellulose oxidation, hydroxyl groups of the glycosides of the cellulosechains can be converted, for example, to carbonyl groups such asaldehyde groups or carboxylic acid groups. Depending on the oxidationmethod and conditions used, the type, degree, and location of thecarbonyl modifications may vary. It is known that certain oxidationconditions may degrade the cellulose chains themselves, for example bycleaving the glycosidic rings in the cellulose chain, resulting indepolymerization. In most instances, depolymerized cellulose not onlyhas a reduced viscosity, but also has a shorter fiber length than thestarting cellulosic material. When cellulose is degraded, such as bydepolymerizing and/or significantly reducing the fiber length and/or thefiber strength, it may be difficult to process and/or may be unsuitablefor many downstream applications. A need remains for methods ofmodifying cellulose fiber that may improve both carboxylic acid andaldehyde functionalities, which methods do not extensively degrade thecellulose fiber.

Various attempts have been made to oxidize cellulose to provide bothcarboxylic and aldehydic functionality to the cellulose chain withoutdegrading the cellulose fiber. In many cellulose oxidation methods, ithas been difficult to control or limit the degradation of the cellulosewhen aldehyde groups are present on the cellulose. Previous attempts atresolving these issues have included the use of multi-step oxidationprocesses, for instance site-specifically modifying certain carbonylgroups in one step and oxidizing other hydroxyl groups in another step,and/or providing mediating agents and/or protecting agents, all of whichmay impart extra cost and by-products to a cellulose oxidation process.Thus, there exists a need for methods of modifying cellulose that arecost effective and/or can be performed in a single step of a process,such as a kraft process.

In addition to the difficulties in controlling the chemical structure ofcellulose oxidation products, and the degradation of those products, itis known that the method of oxidation may affect other properties,including chemical and physical properties and/or impurities in thefinal products. For instance, the method of oxidation may affect thedegree of crystallinity, the hemi-cellulose content, the color, and/orthe levels of impurities in the final product and the yellowingcharacteristics of the fiber. Ultimately, the method of oxidation mayimpact the ability to process the cellulose product for industrial orother applications

After oxidation, cellulose oxidation products can be treated to furtherimprove functionality. Modified cellulose fibers may be treated with acarboxylating acid that converts aldehyde functional groups to carboxylfunctional groups, Carboxylating acids include chlorous acid, acidicpotassium dichromate, and potassium permanganate. Methods of chlorousacid treatment are known. Typically, sodium chlorite or chlorine dioxideis used to form chlorous acid. As chlorous acid is generated, it isquickly converted to hypochlorous acid. Typically, hydrogen peroxide isalso added with sodium chlorite or chlorine dioxide to converthypochlorous acid back to chlorous acid.

Traditionally, cellulose sources that were useful in the production ofabsorbent products or tissue were not also useful in the production ofdownstream cellulose derivatives, such as cellulose ethers and celluloseesters. The production of low viscosity cellulose derivatives from highviscosity cellulose raw materials, such as standard kraft fiber,requires additional manufacturing steps that would add significant costwhile imparting unwanted by-products and reducing the overall quality ofthe cellulose derivative. Cotton linter and high alpha cellulose contentsulfite pulps are typically used in the manufacture of cellulosederivatives such as cellulose ethers and esters. However, production ofcotton linters and sulfite fiber with a high degree of polymerization(DP) and/or viscosity is expensive due to 1) the cost of the startingmaterial, in the case of cotton; 2) the high energy, chemical, andenvironmental costs of pulping and bleaching, in the case of sulfitepulps; and 3) the extensive purifying processes required, which appliesin both cases. In addition to the high cost, there is a dwindling supplyof sulfite pulps available to the market. Therefore, these fibers arevery expensive, and have limited applicability in pulp and paperapplications, for example, where higher purity or higher viscosity pulpsmay be required. For cellulose derivative manufacturers these pulpsconstitute a significant portion of their overall manufacturing cost.Thus, there exists a need for high purity, white, bright, stable againstyellowing, low cost fibers, such as a kraft fiber, that may be used inthe production of cellulose derivatives.

There is also a need for inexpensive cellulose materials that can beused in the manufacture of microcrystalline cellulose. Microcrystallinecellulose is widely used in food, pharmaceutical, cosmetic, andindustrial applications, and is a purified crystalline form of partiallydepolymerized cellulose. The use of kraft fiber in microcrystallinecellulose production, without the addition of extensive post-bleachingprocessing steps, has heretofore been limited. Microcrystallinecellulose production generally requires a highly purified cellulosicstarting material, which is acid hydrolyzed to remove amorphous segmentsof the cellulose chain. See U.S. Pat. No. 2,978,446 to Battista et al.and U.S. Pat. No. 5,346,589 to Braunstein et al. A low degree ofpolymerization of the chains upon removal of the amorphous segments ofcellulose, termed the “level-off DP,” is frequently a starting point formicrocrystalline cellulose production and its numerical value dependsprimarily on the source and the processing of the cellulose fibers. Thedissolution of the non-crystalline segments from standard kraft fibergenerally degrades the fiber to an extent that renders it unsuitable formost applications because of at least one of 1) remaining impurities; 2)a lack of sufficiently long crystalline segments; or 3) it results in acellulose fiber having too high a degree of polymerization, typically inthe range of 200 to 400, to make it useful in the production ofmicrocrystalline cellulose. Kraft fiber having an increased alphacellulose content, for example, would be desirable, as the kraft fibermay provide greater versatility in microcrystalline cellulose productionand applications.

The methods of the present disclosure result in products that havecharacteristics that are not seen in prior art fibers. Thus, the methodsof the disclosure can be used to produce products that are superior toproducts of the prior art. In addition, the fiber of the presentinvention can be cost-effectively produced.

DESCRIPTION

I. Methods

The present disclosure provides novel methods for producing cellulosefiber. The method comprises subjecting cellulose to a krait pulpingstep, an oxygen delignification step, a bleaching sequence andcarboxylating acid treatment. Similar pulping and bleaching processesare disclosed in published International Applications Nos. WO2010/138941, and WO 12012/170183, which are incorporated by reference intheir entirety. Fiber produced under the conditions as described in theinstant application exhibits the same high whiteness and high brightnesswhile having an enhanced carboxyl content over the fiber described inpublished International Application Serial No. WO 2010/138941.

The present disclosure provides novel methods for producing cellulosefiber. The method comprises subjecting cellulose to a kraft pulpingstep, an oxygen delignification step, a bleaching sequence whichincludes at least one catalytic oxidation stage and carboxylating acidtreatment. In one embodiment, the conditions under which the celluloseis processed result in softwood fiber exhibiting high brightness and lowviscosity while reducing the tendency of the fiber to yellow uponexposure to heat, light and/or chemical treatment.

The cellulose fiber used in the methods described herein may be derivedfrom softwood fiber, hardwood fiber, and mixtures thereof. In someembodiments, the cellulose fiber is derived from softwood, such assouthern pine. In some embodiments, the cellulose fiber is derived fromhardwood, such as eucalyptus. In some embodiments, the cellulose fiberis derived from a mixture of softwood and hardwood. In yet anotherembodiment, the cellulose fiber is derived from cellulose fiber that haspreviously been subjected to all or part of a kraft process, i.e., kraftfiber.

References in this disclosure to “cellulose fiber,” “kraft fiber,” “pulpfiber” or “pulp” are interchangeable except where specifically indicatedto be different or where one of ordinary skill in the art wouldunderstand them to be different. As used herein “modified acid-treatedkraft fiber,” i.e., fiber which has been cooked, bleached, oxidized andtreated with a carboxylating acid in accordance with the presentdisclosure may be used interchangeably with “kraft fiber” or “pulpfiber” to the extent that the context warrants it.

The present disclosure provides novel methods for treating cellulosefiber. In some embodiments, the disclosure provides a method ofmodifying cellulose fiber, comprising providing cellulose fiber, andoxidizing the cellulose fiber, resulting in modified cellulose fiber Asused herein, “oxidized,” “catalytically oxidized” “catalytic oxidation”and “oxidation” are all understood to be interchangeable and refer totreatment of cellulose fiber with at least one metal catalyst, such asiron or copper and at least one peroxide, such as hydrogen peroxide,such that at least some of the hydroxyl groups of the cellulose fibersare oxidized. The phrase “iron or copper” and similarly “iron (orcopper)” mean “iron or copper or a combination thereof.” In someembodiments, the oxidation comprises simultaneously increasingcarboxylic acid and aldehyde content of the cellulose fiber.

In some embodiments, the disclosure provides a method of treatingmodified cellulose fiber with a carboxylating acid, comprising providingmodified fiber, and acid treating the modified cellulose fiber. As usedherein, “acid-treated” refers to treatment of modified cellulose fiberwith at least one acid, such as chlorous acid, such that at least someof the aldehyde groups of the modified cellulose fibers are converted tocarboxyl groups.

In one embodiment, cellulose is digested using any method that is knownin the art. A typical method of digestion includes the removal of ligninfrom cellulose fiber in hot alkaline solution. This process is oftenreferred to as “cooking.” Typically, the white liquor is an alkalineaqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na₂S).Generally, the temperature of the wood/liquor mixture in the digester ismaintained at about 145° C. to 170° C. for a total reaction time ofabout 1-3 hours. When digestion is complete, the resulting kraft woodpulp is separated from the spent liquor (black liquor) which includesthe used chemicals and dissolved lignin.

Digestion may be carried out with our without oxygen delignification.The typical Kappa number (the measure used to determine the amount ofresidual lignin in pulp) of the pulp after cooking, and optionallyoxygen delignification, and prior to bleaching is in the range of 28 to32.

According to another embodiment, preferably southern pine, is digestedin a two-vessel hydraulic digester with, Lo-Solids® cooking to a kappanumber ranging from about 10 to about 21. The resulting pulp issubjected to oxygen delignification until it reaches a kappa number ofabout 8 or below, for example, 6.5 or below. The cellulose pulp is thenbleached in a multi-stage bleaching sequence which includes at least onecatalytic oxidation stage.

In one embodiment, the method comprises digesting the cellulose fiber ina continuous digester with a co-current, down-flow arrangement. Theeffective alkali (“EA”) of the white liquor charge is at least about 15%on pulp, for example, at least about 15.5% on pulp, for example at leastabout 16% on pulp, for example, at least about 16.4% on pulp, forexample at least about 17% on pulp, for example at least about 18% onpulp, for example, at least about 18.5% on pulp. As used herein a “% onpulp” refers to an amount based on the dry weight of the kraft pulp. Inone embodiment, the white liquor charge is divided with a portion of thewhite liquor being applied to the cellulose in the impregnator and theremainder of the white liquor being applied to the pulp in the digester.According to one embodiment, the white liquor is applied in a 50:50ratio. In another embodiment, the white liquor is applied in a range offrom 90:10 to 30:70, for example in a range from 50:50 to 70:30, forexample 60:40. According to one embodiment, the white liquor is added tothe digester in a series of stages. According to one embodiment,digestion is carried out at a temperature between about 160° C. to about168° C., for example, from about 163° C. to about 168° C., for example,from about 166° C. to about 168° C., and the cellulose is treated untila target kappa number between about 17 and about 21 is reached. It isbelieved that the higher than normal effective alkali (“EA”) and highertemperatures than used in the prior art achieve the lower than normalKappa number.

According to one embodiment of the invention, the digester is run withan increase in push flow which increases the liquid to wood ratio as thecellulose enters the digester. This addition of white liquor is believedto assist in maintaining the digester at a hydraulic equilibrium andassists in achieving a continuous down-flow condition in the digester.

In one embodiment, the method comprises oxygen delignifying thecellulose fiber after it has been cooked to a kappa number from about 13to about 21 to further reduce the lignin content and further reduce thekappa number, prior to bleaching. Oxygen delignification can beperformed by any method known to those of ordinary skill in the art. Forinstance, oxygen delignification may be carried out in a conventionaltwo-stage oxygen delignification process. Advantageously, thedelignification is carried out to a target kappa number of about 8 orlower, for example about 6.5 or lower, for example about 5 to about 8.

In one embodiment, during oxygen delignification, the applied oxygen isless than about 3% on pulp, for example, less than about 2.4% on pulp,for example, less than about 2% on pulp, for example less than about1.8% on pulp, for example less than about 1.6% on pulp. According to oneembodiment, fresh caustic is added to the cellulose during oxygendelignification. Fresh caustic may be added in an amount of from about2% on pulp to about 3.8% on pulp, for example, from about 3% on pulp toabout 3.2% on pulp. According to one embodiment, the ratio of oxygen tocaustic is reduced over standard kraft production; however the absoluteamount of oxygen remains the same. Delignification may be carried out ata temperature of from about 85° C. to about 104° C., for example, fromabout 90° C. to about 102° C., for example, from about 96° C. to about102° C., for example about 90° C. to about 96° C.

After the fiber has reached the desired Kappa Number of about 8 or less,for example, 6.5 or less, the fiber is subjected to a multi-stagebleaching sequence. The stages of the multi-stage bleaching sequence mayinclude any conventional or after discovered series of stages and may beconducted under conventional conditions

In some embodiments, prior to bleaching the pH of the cellulose isadjusted to a pH ranging from about 2 to about 6, for example from about2 to about 5 or from about 2 to about 4, or from about 2 to about 3.

The pH can be adjusted using any suitable acid, as a person of skillwould recognize, for example, sulfuric acid or hydrochloric acid orfiltrate from an acidic bleach stage of a bleaching process, such as achlorine dioxide (D) stage of a multi-stage bleaching process. Forexample, the cellulose fiber may be acidified by adding an extraneousacid, Examples of extraneous acids are known in the art and include, butare not limited to, sulfuric acid, hydrochloric acid, and carbonic acid.In some embodiments, the cellulose fiber is acidified with acidicfiltrate, such as waste filtrate, from a bleaching step. In at least oneembodiment, the cellulose fiber is acidified with acidic filtrate from aD stage of a multi-stage bleaching process.

The fiber, described, is subjected to a catalytic oxidation treatment.In some embodiments, the fiber is oxidized with iron and/or copper and aperoxide.

Oxidation of cellulose fiber involves treating the cellulose fiber withat least a catalytic amount of a metal catalyst, such as iron or copperand a peroxygen, such as hydrogen peroxide. In at least one embodiment,the method comprises oxidizing cellulose fiber with iron and hydrogenperoxide. The source of iron can be any suitable source, as a person ofskill would recognize, such as for example ferrous sulfate (for exampleferrous sulfate heptahydrate), ferrous chloride, ferrous ammoniumsulfate, ferric chloride, ferric ammonium sulfate, or ferric ammoniumcitrate.

In some embodiments, the method comprises oxidizing the cellulose fiberwith copper and hydrogen peroxide. Similarly, the source of copper canbe any suitable source as a person of skill would recognize. Finally, insome embodiments, the method comprises oxidizing the cellulose fiberwith a combination of copper and iron and hydrogen peroxide.

When cellulose fiber is oxidized, it is done in an acidic environment.The fiber should not be subjected to substantially alkaline conditionsduring the oxidation. In some embodiments, the method comprisesoxidizing cellulose fiber at an acidic pH. In some embodiments, themethod comprises providing cellulose fiber, acidifying the cellulosefiber, and then oxidizing the cellulose fiber at acidic pH. In someembodiments, the pH ranges from about 2 to about 6, for example fromabout 2 to about 5 or from about 2 to about 4.

In some embodiments, the method comprises oxidizing the cellulose fiberin one or more stages of a multi-stage bleaching sequence. In someembodiments, the cellulose fiber may be oxidized in either the secondstage or the fourth stage of a multi-stage bleaching sequence, forexample, a five-stage bleaching sequence. In other embodiments, theoxidation may be carried out in two or more stages of a multi-stagebleaching sequence. In some embodiments, the cellulose fiber may befurther oxidized in one or more additional stages preceding or followingthe bleaching sequence.

In accordance with the disclosure, the multi-stage bleaching sequencecan be any bleaching sequence. In at least one embodiment, themulti-stage bleaching sequence is a five-stage bleaching sequence. Insome embodiments, the bleaching sequence is a DEDED sequence. In someembodiments, the bleaching sequence is a D₀E1D1E2D2 sequence. In someembodiments, the bleaching sequence is a D₀(EoP)D1E2D2 sequence. In someembodiments the bleaching sequence is a D₀(EO)D1E2D2.

The non-oxidation stages of a multi-stage bleaching sequence may includeany conventional or after discovered series of stages and may beconducted under conventional conditions. In some embodiments, theoxidation is incorporated into the fourth stage of a multi-stagebleaching process. In some embodiments, the method is implemented in afive-stage bleaching process having a sequence of D₀E1D1E2D2, and thefourth stage (E2) is used for oxidizing kraft fiber.

In some embodiments, the oxidation is incorporated into the second stageof a multi-stage bleaching process. In some embodiments, the method isimplemented in a five-stage bleaching process having a sequence ofD₀E1D1E2D2, and the second stage (E1) is used for oxidizing kraft fiber.

In some embodiments, the oxidation is incorporated into two or morestages of a multi-stage bleaching process. In some embodiments, themethod is implemented in a five-stage bleaching process having asequence of D₀E1D1E2D2, and the second and fourth stages (E2) are usedfor oxidizing kraft fiber.

In some embodiments, the kappa number increases after oxidation of thecellulose fiber. More specifically, one would typically expect adecrease in kappa number across an oxidation bleaching stage based uponthe anticipated decrease in material, such as lignin, which reacts withthe permanganate reagent. However, in the method as described herein,the kappa number of cellulose fiber may decrease because of the loss ofimpurities, e.g., lignin; however, the kappa number may increase becauseof the chemical modification of the fiber. Not wishing to be bound bytheory, it is believed that the increased functionality of the modifiedcellulose provides additional sites that can react with the permanganatereagent. Accordingly, the kappa number of modified kraft fiber iselevated relative to the kappa number of standard kraft fiber.

An appropriate retention time in one or more oxidation stages is anamount of time that is sufficient to catalyze the hydrogen peroxide withthe iron or copper. Such time will be easily ascertainable by a personof ordinary skill in the art.

In accordance with the disclosure, the oxidation is carried out for atime and at a temperature that is sufficient to produce the desiredcompletion of the reaction. For example, the oxidation may be carriedout at a temperature ranging from about 60 to about 90° C., and foratime ranging from about 40 to about 80 minutes. The desired time andtemperature of the oxidation reaction will be readily ascertainable by aperson of skill in the art.

The fiber of the present disclosure may be subjected to any traditionalbleaching sequence using art recognized conditions. The bleachingconditions provided herein are merely exemplary.

According to one embodiment, the cellulose is subjected to a D(EoP)DE2Dbleaching sequence. According to this embodiment, the first D stage (D₀)of the bleaching sequence is carried out at a temperature of at leastabout 57° C., for example at least about 60° C., for example, at leastabout 66° C., for example, at least about 71° C. and at a pH of lessthan about 3, for example about 2.5. Chlorine dioxide is applied in anamount of greater than about 0.6% on pulp, for example, greater thanabout 0.8% on pulp, for example about 0.9% on pulp. Acid is applied tothe cellulose in an amount sufficient to maintain the pH, for example,in an amount of at least about 1% on pulp, for example, at least about1.15% on pulp, for example, at least about 1.25% on pulp.

According to one embodiment, the first E stage (E₁), is carried out at atemperature of at least about 74° C., for example at least about 77° C.,for example at least about 79° C., for example at least about 82° C.,and at a pH of greater than about 11, for example, greater than 11.2,for example about 11.4. Caustic is applied in an amount of greater thanabout 0.7% on pulp, for example, greater than about 0.8% on pulp, forexample about 1.0% on pulp. Oxygen is applied to the cellulose in anamount of at least about 0.48% on pulp, for example, at least about 0.5%on pulp, for example, at least about 0.53% on pulp. Hydrogen Peroxide isapplied to the cellulose in an amount of at least about 0.35% on pulp,for example at least about 0.37% on pulp, for example, at least about0.38% on pulp, for example, at least about 0.4% on pulp, for example, atleast about 0.45% on pulp. The skilled artisan would recognize that anyknown peroxygen compound could be used to replace some or all of thehydrogen peroxide.

According to another embodiment, if the oxidation is carried out in theE₁ stage (E₁), it may be carried out at a temperature of at least about75° C., for example at least about 80° C., for example, at least about82° C. and at a pH of less than about 3.5, for example, less than 3.0,for example, less than about 2.8. An iron catalyst is added in, forexample, aqueous solution at a rate of from about 25 to about 200 ppmFe⁺², for example, from 50 to 150 ppm, for example from 25 to 150 ppm,iron on pulp. Hydrogen Peroxide can be applied to the cellulose at a lowlevel, i.e., in an amount of less than about 0.5% on pulp, for example,less than about 0.3% on pulp, for example, about 0.25% on pulp.Alternatively, the hydrogen peroxide may be applied at a high level,i.e., 1% or more on pulp, for example, 1.5% or more on pulp, forexample, 2.0% or more on pulp. The skilled artisan would recognize thatany known peroxygen compound could be used to replace some or all of thehydrogen peroxide.

According to one embodiment, the second D stage (D₁) of the bleachingsequence is carried out at a temperature of at least about 74° C., forexample at least about 77° C., for example, at least about 79° C., forexample, at least about 82° C. and at a pH of less than about 4, forexample less than 3.5, for example less than 3.2. Chlorine dioxide isapplied in an amount of less than about 1% on pulp, for example, lessthan about 0.8% on pulp, for example about 0.7% on pulp. Caustic isapplied to the cellulose in an amount effective to adjust to the desiredpH, for example, in an amount of less than about 0.015% on pulp, forexample, less than about 0.01% pulp, for example, about 0.0075% on pulp.The TAPPI viscosity of the pulp after this bleaching stage may be 9-12mPa·s, for example or may be lower, for example 6.5 mPa·s or less.

According to one embodiment, the second E stage (E₂), is carried out ata temperature of at least about 74° C., for example at least about 77°C., for example at least about 79° C., for example at least about 82°C., and at a pH of greater than about 11, for example, greater than11.2, for example about 11.4. Caustic is applied in an amount of greaterthan about 0.7% on pulp, for example, greater than about 0.8% on pulp,for example greater than about 1.0% on pulp, for example, greater than1.2% on pulp. Hydrogen Peroxide is applied to the cellulose in an amountof at least about 0.25% on pulp, for example at least about 0.28% onpulp, for example, about 3.0% on pulp. The skilled artisan wouldrecognize that any known peroxygen compound could be used to replacesome or all of the hydrogen peroxide.

According to one embodiment, if the second E stage (E₂), is theoxidation stage, is carried out at a temperature of at least about 74°C., for example at least about 79° C. and at a pH of greater than about2.5, for example, greater than 2.9, for example about 3.3. An ironcatalyst is added in, for example, aqueous solution at a rate of fromabout 25 to about 200 ppm Fe⁺², for example, from 50 to 150 ppm, forexample, from 50 to 100 ppm, iron on pulp. Hydrogen Peroxide can beapplied to the cellulose at a low level, i.e., in an amount of less thanabout 0.5% on pulp, for example, less than about 0.3% on pulp, forexample, about 0.25% on pulp. Alternatively, the hydrogen peroxide maybe applied at a high level, i.e., 1% or more on pulp, for example, 1.5%or more on pulp, for example, 2.0% or more on pulp. The skilled artisanwould recognize that any known peroxygen compound could be used toreplace some or ail of the hydrogen peroxide.

In accordance with the disclosure, hydrogen peroxide is added to thecellulose fiber in acidic media in an amount sufficient to achieve thedesired oxidation and/or degree of polymerization and/or viscosity ofthe final cellulose product. For example, peroxide can be added as asolution at a concentration from about 1% to about 50% by weight in anamount of from about 0.1 to about 0.5%, or from about 0.1% to about0.3%, or from about 0.1% to about 0.2%, or from about 0.2% to about0.3%, based on the dry weight of the pulp.

Iron or copper are added at least in an amount sufficient to catalyzethe oxidation of the cellulose with peroxide. For example, iron can beadded in an amount ranging from about 25 to about 200 ppm based on thedry weight of the kraft pulp, for example, from 50 to 200 ppm, forexample, from 50 to 150 ppm. A person of skill in the art will be ableto readily optimize the amount of iron or copper to achieve the desiredlevel or amount of oxidation and/or degree of polymerization and/orviscosity of the final cellulose product.

In some embodiments, the method further involves adding heat, such asthrough steam, either before or after the addition of hydrogen peroxide.

In some embodiments, the final DP and/or viscosity of the pulp can becontrolled by the amount of iron or copper and hydrogen peroxide and therobustness of the bleaching conditions prior to the oxidation step. Aperson of skill in the art will recognize that other properties of themodified kraft fiber of the disclosure may be affected by the amounts ofcatalyst and peroxide and the robustness of the bleaching conditionsprior to the oxidation step. For example, a person of skill in the artmay adjust the amounts of iron or copper and hydrogen peroxide and therobustness of the bleaching conditions prior to the oxidation step totarget or achieve a desired brightness in the final product and/or adesired degree of polymerization or viscosity.

In some embodiments, a kraft pulp is acidified on a D1 stage washer, theiron source (or copper source) is also added to the kraft pulp on the D1stage washer, the peroxide is added following the iron source (or coppersource) at an addition point in the mixer or pump before the E2 stagetower, the kraft pulp is reacted in the E2 tower and washed on the E2washer, and steam may optionally be added before the E2 tower in a steammixer.

In some embodiments, iron (or copper) can be added up until the end ofthe D1 stage, or the iron (or copper) can also be added at the beginningof the E2 stage, provided that the pulp is acidified first (i.e., priorto addition of the iron (or copper)) at the D1 stage. Steam may beoptionally added either before or after the addition of the peroxide.

For example, in some embodiments, the treatment with hydrogen peroxidein an acidic media with iron (or copper) may involve adjusting the pH ofthe kraft pulp to a pH ranging from about 2 to about 5, adding a sourceof iron (or copper) to the acidified pulp, and adding hydrogen peroxideto the kraft pulp.

According to one embodiment, the third D stage (D₂) of the bleachingsequence is carried out at a temperature of at least about 74° C., forexample at least about 77° C., for example, at least about 79° C., forexample, at least about 82° C. and at a pH of less than about 4, forexample less than about 3.8. Chlorine dioxide is applied in an amount ofless than about 0.5% on pulp, for example, less than about 0.3% on pulp,for example about 0.15% on pulp.

In some embodiments, the method comprises oxidizing the cellulose fiberin two or more stages of a multi-stage bleaching sequence. In otherembodiments, the oxidation may be carried out in two stages chosen fromone or more oxidation stages before the first bleaching stage, one ormore oxidation stages within the bleaching sequence, and oxidation in astage following the bleaching stage. In some embodiments, the cellulosefiber may be oxidized in both the second stage and the fourth stage of amulti-stage bleaching sequence, for example, a five-stage bleachingsequence. In some embodiments, the cellulose fiber may be furtheroxidized in one or more additional stages before or following thebleaching sequence. The non-oxidation stages of a multi-stage bleachingsequence may include any conventional or after discovered series ofstages and may be conducted under conventional conditions. In someembodiments, the oxidation is incorporated into the second and fourthstages of a multi-stage bleaching process. In some embodiments, themethod is implemented in a five-stage bleaching process having asequence of D₀E1D1E2D2, wherein the second (E1) and fourth stage (E2)are used for oxidizing kraft fiber. According to some embodiments, likethe one described, the bleaching sequence does not have any alkalinestages. Therefore, in some embodiments, the present process is an acidicbleaching sequence. Further, contrary to what the art predicts, theacidic bleaching sequence does not suffer from a substantial loss ofbrightness and whiteness.

Alternatively, the multi-stage bleaching sequence may be altered toprovide more robust bleaching conditions prior to oxidizing thecellulose fiber. In some embodiments, the method comprises providingmore robust bleaching conditions prior to the oxidation step. Morerobust bleaching conditions may allow the degree of polymerizationand/or viscosity of the cellulose fiber to be reduced in the oxidationstep with lesser amounts of iron or copper and/or hydrogen peroxide.Thus, it may be possible to modify the bleaching sequence conditions sothat the brightness and/or viscosity of the final cellulose product canbe further controlled. For instance, reducing the amounts of peroxideand metal, while providing more robust bleaching conditions beforeoxidation, may provide a product with lower viscosity and higherbrightness than an oxidized product produced with identical oxidationconditions but with less robust bleaching. Such conditions may beadvantageous in some embodiments, particularly in cellulose etherapplications.

In some embodiments, for example, the method of preparing a modifiedcellulose fiber within the scope of the disclosure may involveacidifying the kraft pulp to a pH ranging from about 2 to about 5 (usingfor example sulfuric acid), mixing a source of iron (for example ferroussulfate, for example ferrous sulfate heptahydrate) with the acidifiedkraft pulp at an application of from about 25 to about 250 ppm Fe⁺²based on the dry weight of the kraft pulp at a consistency ranging fromabout 1% to about 15% and also hydrogen peroxide, which can be added asa solution at a concentration of from about 1% to about 50% by weightand in an amount ranging from about 0.1% to about 2.0% based on the dryweight of the kraft pulp. In some embodiments, the ferrous sulfatesolution is mixed with the kraft pulp at a consistency ranging fromabout 7% to about 15%. In some embodiments the acidic kraft pulp ismixed with the iron source and reacted with the hydrogen peroxide for atime period ranging from about 40 to about 240 minutes at a temperatureranging from about 60 to about 80° C.,

In some embodiments, each stage of the five-stage bleaching processincludes at least a mixer, a reactor, and a washer (as is known to thoseof skill in the art).

The modified fiber, described, is subjected to carboxylating acidtreatment, such as chlorous acid treatment. Chlorous acid treatment ofmodified cellulose fiber involves treating the modified cellulose fiberwith a chlorous acid source, such as sodium chlorite and hydrogenperoxide or chlorine dioxide and hydrogen peroxide. In at least oneembodiment, the method comprises acid treating modified cellulose fiberwith sodium chlorite and hydrogen peroxide.

In some embodiments, the method comprises acid treating the modifiedcellulose fiber in one or more stages of a multi-stage bleachingsequence. In some embodiments, the modified cellulose fiber may be acidtreated in the fifth stage of a multi-stage bleaching sequence, forexample, a five-stage bleaching sequence. In some embodiments, themodified cellulose fiber may be acid treated in one or more additionalstages following the bleaching sequence.

In accordance with the disclosure, the multi-stage bleaching sequencecan be any bleaching sequence. In at least one embodiment, themulti-stage bleaching sequence is a five-stage bleaching sequence. Thefiber of the present disclosure may be subjected to any traditionalbleaching sequence using art recognized conditions.

In some embodiments, the carboxyl context increases after acid treatmentof the cellulose fiber. It is believed that the carboxylating acidtreatment converts aldehyde functional groups to carboxyl functionalgroups. Accordingly, the carboxyl content of modified acid-treated kraftfiber is elevated relative to the carboxyl content of prior modifiedkraft fiber.

In accordance with the disclosure, the acid treatment is carried out fora time and at a temperature that is sufficient to produce the desiredcompletion of the reaction. For example, the acid treatment may becarried out at a temperature about 80° C., and for a time ranging fromabout 120 to about 150 minutes. The desired time and temperature of theacid treatment will be readily ascertainable by a person of skill in theart.

In accordance with the disclosure, a carboxylating acid source is addedto the modified cellulose fiber in an amount sufficient to achieve thedesired carboxyl functionality of the final cellulose product. Forexample, sodium chlorite can be added to the modified cellulose fiber asa source of chlorous acid, as a solution at a concentration from about1.6% to about 2.4% by weight based on the dry weight of the pulp, withhydrogen peroxide added at a concentration from about 0.6% to 0.8% byweight based on the dry weight of the pulp.

According to one embodiment, the third D stage (D2) of the bleachingsequence is chlorous acid treatment. The D2 stage is carried out at atemperature of at least about 80° C. for a time period ranging fromabout 120 to 150 minutes and at a pH of less than 3. Sodium chlorite isadded as a solution at a concentration from about 1.6% to about 2.4% byweight based on the dry weight of the pulp, with hydrogen peroxide addedat a concentration from about 0.6% to 0.8% by weight based on the dryweight of the pulp. The carboxyl content of the modified acid-treatedpulp after this acid treatment stage may be about 7.2 to 7.8 meq/100 g,with an aldehyde content of about 0.3 to 0.6 meq/100 g.

Fiber produced as described may, in some embodiments, be treated with asurface active agent. The surface active agent for use in the presentinvention may be solid or liquid. The surface active agent can be anysurface active agent, including by not limited to softeners, debonders,and surfactants that is not substantive to the fiber, i.e., which doesnot interfere with its specific absorption rate. As used herein asurface active agent that is “not substantive” to the fiber exhibits anincrease in specific absorption rate of 30% or less as measured usingthe pfi test as described herein. According to one embodiment, thespecific absorption rate is increased by 25% or less, such as 20% orless, such as 15% or less, such as 10% or less. Not wishing to be boundby theory, the addition of surfactant causes competition for the samesites on the cellulose as the test fluid. Thus, when a surfactant is toosubstantive, it reacts at too many sites reducing the absorptioncapability of the fiber.

As used herein PFI is measured according to SCAN-C-33:80 Test Standard,

Scandinavian Pulp, Paper and Board Testing Committee. The method isgenerally as follows. First, the sample is prepared using a PFI PadFormer. Turn on the vacuum and feed approximately 3.01 g fluff pulp intothe pad former inlet. Turn off the vacuum, remove the test piece andplace it on a balance to check the pad mass. Adjust the fluff mass to3.00±0.01 g and record as Mass_(dry). Place the fluff into the testcylinder. Place the fluff containing cylinder in the shallow perforateddish of an Absorption Tester and turn the water valve on. Gently apply a500 g load to the fluff pad while lifting the test piece cylinder andpromptly press the start button. The Tester will fun for 30 s before thedisplay will read 00.00. When the display reads 20 seconds, record thedry pad height to the nearest 0.5 mm (Height_(dry)). When the displayagain reads 00.00, press the start button again to prompt the tray toautomatically raise the water and then record the time display(absorption time, T). The Tester will continue to run for 30 seconds.The water tray will automatically lower and the time will run foranother 305. When the display reads 20 s, record the wet pad height tothe nearest 0.5 mm (Height_(wet)). Remove the sample holder, transferthe wet pad to the balance for measurement of Mass_(wet) and shut offthe water valve, Specific Absorption Rate (sig) is T/Mass_(dry).Specific Capacity (g/g) is (Mass_(wet)−Mass_(dry))/Mass_(dry). Wet Bulk(cc/g) is [19.64 cm²×Height_(wet)/3]/10. Dry Bulk is [19.64cm²×Height_(dry)/3]/10. The reference standard for comparison with thesurfactant treated fiber is an identical fiber without the addition ofsurfactant.

It is generally recognized that softeners and debonders are oftenavailable commercially only as complex mixtures rather than as singlecompounds. While the following discussion will focus on the predominantspecies, it should be understood that commercially available mixtureswould generally be used in practice. Suitable softener, debonder andsurfactants will be readily apparent to the skilled artisan and arewidely reported in the literature.

Suitable surfactants include cationic surfactants, anionic, and nonionicsurfactants that are not substantive to the fiber. According to oneembodiment, the surfactant is a non-ionic surfactant. According to oneembodiment, the surfactant is a cationic surfactant. According to oneembodiment, the surfactant is a vegetable based surfactant, such as avegetable based fatty acid, such as a vegetable based fatty acidquaternary ammonium salt. Such compounds include DB999 and DB1009, bothavailable from Cellulose Solutions. Other surfactants may be including,but not limited to Bard 388 an ethoxylated nonylphenol ether from AkzoNobel.

Biodegradable softeners can be utilized. Representative biodegradablecationic softeners/debonders are disclosed in U.S. Pat. Nos. 5,312,522;5,415,737; 5,262,007; 5,264,082; and 5,223,096, all of which areincorporated herein by reference in their entirety. The compounds arebiodegradable diesters of quaternary ammonia compounds, quaternizedamine-esters, and biodegradable vegetable oil based esters functionalwith quaternary ammonium chloride and diester dierucyldimethyl ammoniumchloride and are representative biodegradable softeners.

The surfactant is added in an amount of up to 6 lbs/ton, such as from0.5 lbs/ton to 3 lbs/ton, such as from 0.5 lbs/ton to 2.5 lbs/ton suchas from 0.5 lbs/ton to 2 lbs/ton, such as less than 2 lbs/ton.

The surface active agent may be added at any point prior to formingrolls, bales, or sheets of pulp. According to one embodiment, thesurface active agent is added just prior to the headbox of the pulpmachine, specifically at the inlet of the primary cleaner feed pump.

According to one embodiment, the fiber of the present invention has animproved filterability over the same fiber without the addition ofsurfactant when utilized in a viscose process. For example, thefilterability of a viscose solution comprising fiber of the inventionhas a filterability that is at least 10% lower than a viscose solutionmade in the same way with the identical fiber without surfactant, suchas at least 15% lower, such as at least 30% lower, such as at least 40%lower. Filterability of the viscose solution is measured by thefollowing method. A solution is placed in a nitrogen pressurized (27psi) vessel with a 1 and 3/16ths inch filtered orifice on the bottom—thefilter media is as follows from outside to inside the vessel: aperforated metal disk, a 20 mesh stainless steel screen, muslin cloth, aWhatman 54 filter paper and a 2 layer knap flannel with the fuzzy sideup toward the contents of the vessel. For 40 minutes the solution isallowed to filter through the media, then at 40 minutes for anadditional 140 minutes the (so t=0 at 40 minutes) the volume of filteredsolution is measured (weight) with the elapsed time as the X coordinateand the weight of filtered viscose as the Y coordinate—the slope of thisplot is your filtration number. Recordings to be made at 10 minuteintervals. The reference standard for comparison with the surfactanttreated fiber is the identical fiber without the addition of surfactant.

According to one embodiment of the invention, the surfactant treatedfiber of the invention exhibits a limited increase in specificabsorption rate, e.g., less than 30% with a concurrent decrease infilterability, e.g., at least 10%. According to one embodiment, thesurfactant treated fiber has an increased specific absorption rate ofless than 30% and a decreased filterability of at least 20%, such as atleast 30%, such as at least 40%. According to another embodiment, thesurfactant treated fiber has an increased specific absorption rate ofless than 25% and a decreased filterability of at least 10%, such as atleast about 20%, such as at least 30%, such as at least 40%. Accordingto yet another embodiment, the surfactant treated fiber has an increasedspecific absorption rate of less than 20% and a decreased filterabilityof at least 10%, such as at least about 20%, such as at least 30%, suchas at least 40%. According to another embodiment, the surfactant treatedfiber has an increased specific absorption rate of less than 15% and adecreased filterability of at least 10%, such as at least about 20%,such as at least 30%, such as at least 40%. According to still anotherembodiment, the surfactant treated fiber has an increased specificabsorption rate of less than 10% and an decreased filterability of atleast 10%, such as at least about 20%, such as at least 30%, such as atleast 40%.

Heretofore the addition of cationic surfactant to pulp bound for theproduction of viscose was considered detrimental to viscose production.Cationic surfactants attach to the same sites on the cellulose thatcaustic must react with to begin the breakdown of the cellulose fiber.Thus, it has long been thought that cationic materials should not beused as pulp pre-treatments for fibers used in the production ofviscose. Not wishing to be bound by theory it is believed that since thefibers produced according to the present invention differs from priorart fiber in their form, character and chemistry, the cationicsurfactant is not binding in the same manner as it did to prior artfibers. Fiber according to the disclosure, when treated with asurfactant according to the invention separates the fiber in a way thatimproves caustic penetration and filterability. Thus, according to oneembodiment fibers of the present disclosure can be used as a substitutefor expensive cotton or sulfite fiber to a greater extent than eitheruntreated fiber or prior art fiber has been.

In some embodiments, the disclosure provides a method for controllingodor, comprising providing a kraft fiber according to the disclosure,and applying an odorant to the kraft fiber such that the atmosphericamount of odorant is reduced in comparison with the atmospheric amountof odorant upon application of an equivalent amount of odorant to anequivalent weight of standard kraft fiber. In some embodiments thedisclosure provides a method for controlling odor comprising inhibitingbacterial odor generation. In some embodiments, the disclosure providesa method for controlling odor comprising absorbing odorants, such asnitrogenous odorants, onto a kraft fiber. As used herein, “nitrogenousodorants” is understood to mean odorants comprising at least onenitrogen.

In some embodiments, the disclosure provides a method for producingfluff pulp, comprising providing kraft fiber of the disclosure and thenproducing a fluff pulp. For example, the method comprises bleachingkraft fiber in a multi-stage bleaching process, and then forming a fluffpulp. In at least one embodiment, the fiber is not refined after themulti-stage bleaching process.

In some embodiments, the kraft fiber is combined with at least one superabsorbent polymer (SAP). In some embodiments, the SAP may by an odorreductant. Examples of SAP that can be used in accordance with thedisclosure include, but are not limited to, Hysorb™ sold by the companyBASF, Aqua Keep® sold by the company Sumitomo, and FAVOR®, sold by thecompany Evonik.

II. Kraft Fibers

Reference is made herein to “standard,” “conventional,” or“traditional,” kraft fiber, kraft bleached fiber, kraft pulp or kraftbleached pulp. Such fiber or pulp is often described as a referencepoint for defining the improved properties of the present invention. Asused herein, these terms are interchangeable and refer to the fiber orpulp which is identical in composition to and processed in a likestandard manner. As used herein, a standard kraft process includes botha cooking stage and a bleaching stage under art recognized conditions.Standard kraft processing does not include a pre-hydrolysis stage priorto digestion.

Physical characteristics (for example, purity, brightness, fiber lengthand viscosity) of the kraft cellulose fiber mentioned in thespecification are measured in accordance with protocols provided in theExamples section.

In some embodiments, modified acid-treated kraft fiber of the disclosurehas a brightness equivalent to standard kraft fiber. In someembodiments, the modified cellulose fiber has a brightness of at least85, 86, 87, 88, 89, or 90 ISO. In some embodiments, the brightness is nomore than about 92. In some embodiments, the brightness ranges fromabout 85 to about 92, or from about 86 to about 91, or from about 87 toabout 91, or from about 88 to about 91. In some embodiments, thebrightness is greater than about 91.4 or 91.5 ISO. In some embodiments,the brightness ranges from about 90 to about 91.5.

In some embodiments, modified acid-treated cellulose according to thepresent disclosure has an R18 value in the range of from about 79% toabout 86%, for instance R18 has a value of at least about 86%, forexample, 87.5% to 88.2%, for example, at least about 87%, for example,at least about 87.5%, for example at least about 87.8%, for example atleast about 88%.

In some embodiments, modified acid-treated kraft fiber according to thedisclosure has an R10 value ranging from about 72% to about 82%, forexample, at least about 82%, for example, at least about 83%, forexample, at least about 84%, for example, at least about 85%. The R18and R10 content is described in TAPPI T235. R10 represents the residualundissolved material that is left after extraction of the pulp with 10percent by weight caustic and R18 represents the residual amount ofundissolved material left after extraction of the pulp with an 18%caustic solution. Generally, in a 10% caustic solution, hemicelluloseand chemically degraded short chain cellulose are dissolved and removedin solution. In contrast, generally only hemicellulose is dissolved andremoved in an 18% caustic solution. Thus, the difference between the R10value and the R18 value, (ΔR=R18−R10), represents the amount ofchemically degraded short chained cellulose that is present in the pulpsample.

In some embodiments, modified acid-treated cellulose fiber has an S10caustic solubility ranging from about 14% to about 20%, or from about16% to about 19.5%. In some embodiments, modified cellulose fiber has anS18 caustic solubility ranging from less than about 16%, for exampleless than about 14.5%, for example, less than about 12.5%, for example,less than about 12.3%, for example, about 12%.

The present disclosure provides kraft fiber with low and ultra-lowviscosity. Unless otherwise specified, “viscosity” as used herein refersto 0.5% Capillary CED viscosity measured according to TAPPI T230-om99 asreferenced in the protocols.

Unless otherwise specified, “DP” as used herein refers to average degreeof polymerization by weight (DPw) calculated from 0.5% Capillary CEDviscosity measured according to TAPPI T230-om99. See, e.g., J. F.Cellucon Conference in The Chemistry and Processing of Wood and PlantFibrous Materials, p. 155, test protocol 8, 1994 (Woodhead PublishingLtd., Abington Hall, Abinton Cambridge CBI 6AH England, J. F. Kennedy etal. eds.) “Low DP” means a DP ranging from about 1160 to about 1860 or aviscosity ranging from about 7 to about 13 mPa·s. “Ultra low DP” fibersmeans a DP ranging from about 350 to about 1160 or a viscosity rangingfrom about 3 to about 7 mPa·s.

Without wishing to be bound by theory, it is believed that the fiber ofthe present invention presents an artificial Degree of Polymerizationwhen DP is calculated via CED viscosity measured according to TAPPIT230-om99. Specifically, it is believed that the catalytic oxidationtreatment of the fiber of the present invention doesn't break thecellulose down to the extent indicated by the measured DP, but insteadlargely has the effect of opening up bonds and adding substituents thatmake the cellulose more reactive, instead of cleaving the cellulosechain. It is further believed that the CED viscosity test (TAPPIT230-om99), which begins with the addition of caustic, has the effect ofcleaving the cellulose chain at the new reactive sites, resulting in acellulose polymer which has a much higher number of shorter segmentsthan are found in the fiber's pre-testing state. This is confirmed bythe fact that the fiber length is not significantly diminished duringproduction.

In some embodiments, modified acid-treated cellulose fiber has aviscosity ranging from about 3.0 mPa·s to about 6 mPa·s. In someembodiments, the viscosity ranges from about 4.0 mPa·s to about 5.5mPa·s. In some embodiments, the viscosity ranges from about 4.5 mPa·s toabout 5.5 mPa·s. In some embodiments, the viscosity ranges from about5.0 mPa·s to about 5.5 mPa·s. In some embodiments, the viscosity is lessthan 6 mPa·s, less than 5.5 mPa·s, less than 5.0 mPa·s, less than 4.5mPa·s, or less than 3.5 mPa·s.

In some embodiments, kraft fiber of the disclosure is more compressibleand/or embossable than standard kraft fiber. In some embodiments, kraftfiber may be used to produce structures that are thinner and/or havehigher density than structures produced with equivalent amounts ofstandard kraft fiber.

In some embodiments, kraft fiber of the disclosure maintains its fiberlength during the bleaching process.

“Fiber length” and “average fiber length” are used interchangeably whenused to describe the property of a fiber and mean the length-weightedaverage fiber length. Therefore, for example, a fiber having an averagefiber length of 2 mm should be understood to mean a fiber having alength-weighted average fiber length of 2 mm.

In some embodiments, when the kraft fiber is a softwood fiber, thecellulose fiber has an average fiber length, as measured in accordancewith Test Protocol 12, described in the Example section below, that isabout 2 mm or greater. In some embodiments, the average fiber length isno more than about 3.7 mm. In some embodiments, the average fiber lengthis at least about 2.2 mm, about 2.3 mm, about 2.4 mm, about 2.5 mm,about 2.6 mm, about 2.7 mm, about 2.8 mm, about 2.9 mm, about 3.0 mm,about 3.1 mm, about 3.2 mm, about 3.3 mm, about 3.4 mm, about 3.5 mm,about 3.6 mm, or about 3.7 mm. In some embodiments, the average fiberlength ranges from about 2 mm to about 3.7 mm, or from about 2.2 mm toabout 3.7 mm.

In some embodiments, the modified acid-treated kraft fiber of thedisclosure has increased carboxyl content relative to modified kraftfiber subject to catalytic oxidation but not carboxylating acidtreatment and further relative to standard kraft fiber.

In some embodiments, modified acid-treated kraft fiber has a carboxylcontent ranging from about 6 meq/100 g to about 8 meq/100 g. In someembodiments, the carboxyl content ranges from about 6 meg/100 g to about7 meq/100 g. In some embodiments, the carboxyl content is at least about6 meq/100 g, for example, at least about 6.5 meq/100 g, for example, atleast about 7.0 meq/100 g, at least about 7.5 meq/100 g, for example,and at least about 8.0 meq/100 g, for example.

In some embodiments, modified add-treated kraft fiber has an aldehydecontent of less than about 1 meq/100 g, for example, less than about 0.6meq/100 g, for example, ranging from about 0.3 to about 0.6 meq/100 g.In some embodiments, the modified acid-treated kraft fiber of thedisclosure has reduced aldehyde content and increased carboxyl contentrelative to kraft fiber subject to catalytic oxidation but notcarboxylating acid treatment. Aldehyde functionality contributes toyellowing and brightness reversion. In some embodiments, the modifiedacid-treated kraft fiber of the disclosure exhibits reduced yellowingand reduced brightness reversion.

In some embodiments, modified acid-treated kraft fiber of the disclosurehas a carbonyl content ranging from about 2.5 meq/100 g to about 2.6meq/100 g. In some embodiments, the carbonyl content is less than about2.5 meq/100 g.

In some embodiments, the fiber exhibits increased carboxyl content anddecreased aldehyde functionality relative to modified kraft fibersubject to catalytic oxidation but not carboxylating acid treatment andalso relative to standard kraft fiber.

Kraft fiber of the disclosure may be more flexible than standard kraftfiber, and may elongate and/or bend and/or exhibit elasticity and/orincrease wicking. Additionally, it is expected that the kraft fiber ofthe disclosure would be softer than standard kraft fiber, enhancingtheir applicability in absorbent product applications, for example, suchas diaper and bandage applications.

In some embodiments, the modified acid-treated cellulose fiber has acopper number less than about 2. In some embodiments, the copper numberis less than about 1.5. In some embodiments, the copper number is lessthan about 1.3, for example, less than about 0.9.

In at least one embodiment, the hemicellulose content of the modifiedacid-treated kraft fiber is substantially the same as standardunbleached kraft fiber. For example, the hemicellulose content for asoftwood kraft fiber may range from about 12% to about 17%. Forinstance, the hemicellulose content of a hardwood kraft fiber may rangefrom about 12.5% to about 16.5%.

III. Products Made from Kraft Fibers

The present disclosure provides products made from the modifiedacid-treated kraft fiber described herein. In some embodiments, theproducts are those typically made from standard kraft fiber. In otherembodiments, the products are those typically made from cotton linter,pre-hydrolsis kraft or sulfite pulp. More specifically, fiber of thepresent invention can be used, without further modification, in theproduction of absorbent products and as a starting material in thepreparation of chemical derivatives, such as ethers and esters.Heretofore, fiber has not been available which has been useful toreplace both high alpha content cellulose, such as cotton and sulfitepulp, as well as traditional kraft fiber,

Phrases such as “which can be substituted for cotton linter (or sulfitepulp) . . . ” and “interchangeable with cotton linter (or sulfite pulp). . . ” and “which can be used in place of cotton linter (or sulfitepulp) . . . ” and the like mean only that the fiber has propertiessuitable for use in the end application normally made using cottonlinter (or sulfite pulp or pre-hydrolysis kraft fiber). The phrase isnot intended to mean that the fiber necessarily has all the samecharacteristics as cotton linter (or sulfite pulp).

In some embodiments, the disclosure provides a method for producingfluff pulp. For example, the method comprises bleaching kraft fiber in amulti-stage bleaching process, and then forming a fluff pulp. In atleast one embodiment, the fiber is not refined after the multi-stagebleaching process.

In some embodiments, the products are absorbent products, including, butnot limited to, medical devices, including wound care (e.g. bandage),baby diapers nursing pads, adult incontinence products, feminine hygieneproducts, including, for example, sanitary napkins and tampons, air-laidnon-woven products, air-laid composites, “table-top” wipers, napkin,tissue, towel and the like. In some embodiments, the kraft fiber of thepresent invention exhibits improved bacteriostatic properties, making itsuitable for absorbent products in contact with skin. Absorbent productsaccording to the present disclosure may be disposable. In thoseembodiments, fiber according to the invention can be used as a whole orpartial substitute for the bleached hardwood or softwood fiber that istypically used in the production of these products.

In some embodiments, the kraft fiber of the present invention is in theform of fluff pulp and has one or more properties that make the kraftfiber more effective than conventional fluff pulps in absorbentproducts. More specifically, kraft fiber of the present invention mayhave improved compressibility which makes it desirable as a substitutefor currently available fluff pulp fiber. Because of the improvedcompressibility of the fiber of the present disclosure, it is useful inembodiments which seek to produce thinner, more compact absorbentstructures. One skilled in the art, upon understanding the compressiblenature of the fiber of the present disclosure, could readily envisionabsorbent products in which this fiber could be used. By way of example,in some embodiments, the disclosure provides an ultrathin hygieneproduct comprising the kraft fiber of the disclosure. Ultra-thin fluffcores are typically used in, for example, feminine hygiene products orbaby diapers. Other products which could be produced with the fiber ofthe present disclosure could be anything requiring an absorbent core ora compressed absorbent layer. When compressed, fiber of the presentinvention exhibits no or no substantial loss of absorbency, but shows animprovement in flexibility.

The cellulose fibers of the disclosure exhibit antiviral and/orantimicrobial activity. The cellulose fibers of the present inventionare useful in the production of articles that would come into contactwith microbes, viruses or bacteria and thus, would benefit frominhibition of the growth of those infectious agents. Absorbent articlesor devices include bandages, bandaids, medical gauze, absorbentdressings and pads, medical gowning, paper for medical tables, andincontinence pads for hospital use, just to name a few. The fiber of thedisclosure can be included within, e.g., can be a portion of, or canmake-up the entire absorbent portion of the absorbent device.

Fiber of the present invention may, without further modification, alsobe used in the production of absorbent products including, but notlimited to, tissue, towel, napkin and other paper products which areformed on a traditional papermaking machine. Traditional papermakingprocesses involve the preparation of an aqueous fiber slurry which istypically deposited on a forming wire where the water is thereafterremoved. The kraft fibers of the present disclosure may provide improvedproduct characteristics in products including these fibers. In someembodiments, the modified acid-treated fiber of the present inventionmay exhibit improved water absorptive capacity and improved dry and wetstrength, making it suitable for absorbent products, including tissueand towel-type applications.

In some embodiments, the kraft fiber is combined with at least one superabsorbent polymer (SAP). In some embodiments, the SAP may by an odorreductant. Examples of SAP that can be used in accordance with thedisclosure include, but are not limited to, Hysorb™ sold by the companyBASF, Aqua Keep® sold by the company Sumitomo, and FAVOR®, sold by thecompany Evonik.

In some embodiments, the disclosure provides a method for controllingodor, comprising providing a modified acid-treated kraft fiber accordingto the disclosure, and applying an odorant to the kraft fiber such thatthe atmospheric amount of odorant is reduced in comparison with theatmospheric amount of odorant upon application of an equivalent amountof odorant to an equivalent weight of standard kraft fiber. In someembodiments the disclosure provides a method for controlling odorcomprising inhibiting bacterial odor generation. In some embodiments,the disclosure provides a method for controlling odor comprisingabsorbing odorants, such as nitrogenous odorants, onto a modifiedacid-treated kraft fiber. As used herein, “nitrogenous odorants” isunderstood to mean odorants comprising at least one nitrogen.

IV. Acid/Alkaline Hydrolyzed Products

In some embodiments, this disclosure provides a modified acid-treatedkraft fiber that can be used as a substitute for cotton linter orsulfite pulp. In some embodiments, this disclosure provides a modifiedkraft fiber that can be used as a substitute for cotton linter orsulfite pulp, for example in the manufacture of cellulose ethers,cellulose acetates viscose, and microcrystalline cellulose.

Without being bound by theory, it is believed that the increase inaldehyde content relative to conventional kraft pulp provides additionalactive sites for etherification to end-products such ascarboxymethylcellulose, methylcellulose, hydroxypropylcellulose, and thelike, while simultaneously reducing the viscosity and DP withoutimparting significant yellowing or discoloration, enabling production ofa fiber that can be used for both papermaking and cellulose derivatives.

In some embodiments, the modified acid-treated kraft fiber has chemicalproperties that make it suitable for the manufacture of celluloseethers. Thus, the disclosure provides a cellulose ether derived from amodified acid-treated kraft fiber as described. In some embodiments, thecellulose ether is chosen from ethylcellulose, methylcellulose,hydroxypropyl cellulose, carboxymethyl cellulose, hydroxypropylmethylcellulose, and hydroxyethyl methyl cellulose. It is believed thatthe cellulose ethers of the disclosure may be used in any applicationwhere cellulose ethers are traditionally used. For example, and not byway of limitation, the cellulose ethers of the disclosure may be used incoatings, inks, binders, controlled release drug tablets, and films.

In some embodiments, the modified acid-treated kraft fiber has chemicalproperties that make it suitable for the manufacture of celluloseesters. Thus, the disclosure provides a cellulose ester, such as acellulose acetate, derived from modified kraft fibers of the disclosure.In some embodiments, the disclosure provides a product comprising acellulose acetate derived from the modified acid-treated kraft fiber ofthe disclosure. For example, and not by way of limitation, the celluloseesters of the disclosure may be used in, home furnishings, cigarettefilters, inks, absorbent products, medical devices, and plasticsincluding, for example, LCD and plasma screens and windshields.

In some embodiments, the modified acid-treated kraft fiber of thedisclosure may be suitable for the manufacture of viscose. Moreparticularly, the modified acid-treated kraft fiber of the disclosuremay be used as a partial substitute for expensive cellulose startingmaterial. The modified acid-treated kraft fiber of the disclosure mayreplace as much as 15% or more, for example as much as 10%, for exampleas much as 5%, of the expensive cellulose starting materials. Thus, thedisclosure provides a viscose fiber derived in whole or in part from amodified acid-treated kraft fiber as described. In some embodiments, theviscose is produced from modified acid-treated kraft fiber of thepresent disclosure that is treated with alkali and carbon disulfide tomake a solution called viscose, which is then spun into dilute sulfuricacid and sodium sulfate to reconvert the viscose into cellulose. It isbelieved that the viscose fiber of the disclosure may be used in anyapplication where viscose fiber is traditionally used. For example, andnot by way of limitation, the viscose of the disclosure may be used inrayon, cellophane, filament, food casings, and tire cord.

In some embodiments, the modified acid-treated kraft fiber is suitablefor the manufacture of microcrystalline cellulose. Microcrystallinecellulose production requires relatively clean, highly purified startingcellulosic material. As such, traditionally, expensive sulfite pulpshave been predominantly used for its production. The present disclosureprovides microcrystalline cellulose derived from kraft fiber of thedisclosure. Thus, the disclosure provides a cost-effective cellulosesource for microcrystalline cellulose production.

The cellulose of the disclosure may be used in any application thatmicrocrystalline cellulose has traditionally been used. For example, andnot by way of limitation, the cellulose of the disclosure may be used inpharmaceutical or nutraceutical applications, food applications,cosmetic applications, paper applications, or as a structural composite.For instance, the cellulose of the disclosure may be a binder, diluent,disintegrant, lubricant, tabletting aid, stabilizer, texturizing agent,fat replacer, bulking agent, anticaking agent, foaming agent,emulsifier, thickener, separating agent, gelling agent, carriermaterial, opacifier, or viscosity modifier. In some embodiments, themicrocrystalline cellulose is a colloid.

Other products comprising cellulose derivatives and microcrystallinecellulose derived from kraft fibers according to the disclosure may alsobe envisaged by persons of ordinary skill in the art. Such products maybe found, for example, in cosmetic and industrial applications.

Fiber for use in the production of chemical derivatives can be sensitiveto the level of functionality that has been imparted by the oxidationprocess. Specifically, aldehyde groups can be a source of brightnessreversion as the fiber ages. Fiber for use in the production of chemicalderivatives and viscose ideally has a low viscosity and concurrently alow aldehyde content. The addition of oxygen to any of the oxidationstages has little effect on viscosity but materially reduces thealdehyde functionality of the fiber. Further, the fiber does not exhibitan increased carboxyl content. Without wishing to be bound by theory, itis believed that the aldehyde groups are being oxidized to carbondioxide and are released.

Thus, according to one embodiment of the invention oxygen is introducedat one or more of the oxidation stages to reduce the level of aldehydefunctionality. The use of oxygen during the oxidation process can beused to reduce aldehyde content in process where the fiber is latertreated with a carboxylating acid and in processes where it is not.Fiber that has been treated in an oxidation stage that includes oxygencan have an aldehyde content of less than about 4 meq/100 g, forexample, less than 3.5 meq/100 g, for example, less than 3.2 meq/100 g.

The levels of oxygen added to the oxidation stage are from about 0.1% toabout 1%, for example from about 0.3% to about 0.7%, for example, fromabout 0.4% to about 0.5%, for about 0.5% to about 0.6%.

As used herein, “about” is meant to account for variations due toexperimental error. All measurements are understood to be modified bythe word “about”, whether or not “about” is explicitly recited, unlessspecifically stated otherwise. Thus, for example, the statement “a fiberhaving a length of 2 mm” is understood to mean “a fiber having a lengthof about 2 mm.”

The details of one or more non-limiting embodiments of the invention areset forth in the examples below. Other embodiments of the inventionshould be apparent to those of ordinary skill in the art afterconsideration of the present disclosure.

EXAMPLES Test Protocols

-   -   1. Caustic solubility (R10, S10, R18, S18) is measured according        to TAPPI T235-cm00.    -   2. Carboxyl content is measured according to TAPPI T237-cm98.    -   3. Aldehyde content is measured according to Econotech Services        LTD, proprietary procedure ESM 055B.    -   4. Copper Number is measured according to TAPPI T430-cm99.    -   5. Carbonyl content is calculated from Copper Number according        to the formula: carbonyl=(Cu. No.—0.07)/0.6, from        Biomacromolecules 2002, 3, 969-975.    -   6. 0.5% Capillary CED Viscosity is measured according to TAPPI        T230-om99.    -   7. Intrinsic Viscosity is measured according to ASTM D1795        (2007).    -   8. DP is calculated from 0.5% Capillary CED Viscosity according        to the formula: DPw=−449.6+598.4 ln(0.5% Capillary CED)+118.02        ln²(0.5% Capillary CED), from the 1994 Cellucon Conference        published in The Chemistry and Processing Of Wood And Plant        Fibrous Materials, p. 155, woodhead Publishing Ltd, Abington        Hall, Abington, Cambridge CBI 6AH, England, J. F. Kennedy, et        al. editors.    -   9. Carbohydrates are measured according to TAPPI T249-cm00 with        analysis by Dionex ion chromatography.    -   10. Cellulose content is calculated from carbohydrate        composition according to the formula:        Cellulose=Glucan−(Mannan/3), from TAPPI Journal 65(12):78-80        1982.    -   11. Hemicellulose content is calculated from the sum of sugars        minus the cellulose content.    -   12. Fiber length and coarseness is determined on a Fiber Quality        Analyzer™ from OPTEST, Hawkesbury, Ontario, according to the        manufacturer's standard procedures.    -   13. DCM (dichloromethane) extractives are determined according        to TAPPI T204-cm97.    -   14. Iron content is determined by acid digestion and analysis by        ICP.    -   15. Ash content is determined according to TAPPI T211-om02.    -   16. Brightness is determined according to TAPPI T525-om02.    -   17. CIE Whiteness is determined according to TAPPI Method T560

Example 1 (Fourth Stage Oxidation)

Methods of Preparing Fibers of the Disclosure

Southern pine chips were cooked in a two vessel continuous digester withLo-Solids® downflow cooking. The white liquor application was 8.42% aseffective alkali (EA) in the impregnation vessel and 8.59% in the quenchcirculation. The quench temperature was 166° C. The kappa no. afterdigesting was 20.4. The brownstock pulp was further delignified in a twostage oxygen delignification system with 2.98% sodium hydroxide (NaOH)and 2.31% oxygen (O₂) applied. The temperature was 98° C. The firstreactor pressure was 758 kPa and the second reactor was 372 kPa. Thekappa no. was 6.95.

The oxygen delignified pulp was bleached in a 5 stage bleach plant. Thefirst chlorine dioxide stage (D0) was carried out with 0.90% chlorinedioxide (ClO₂) applied at a temperature of 61° C. and a pH of 2.4.

The second or oxidative alkaline extraction stage (EOP) was carried outat a temperature of 76° C. NaOH was applied at 0.98%, hydrogen peroxide(H₂O₂) at 0.44%, and oxygen (O₂) at 0.54%. The kappa no. after oxygendelignification was 2.1

The third or chlorine dioxide stage (D1) was carried out at atemperature of 74° C. and a pH of 3.3. ClO₂ was applied at 0.61% andNaOH at 0.02%, The 0.5% Capillary CED viscosity was 10.0 mPa·s.

The fourth stage was altered to produce a low degree of polymerizationpulp. Ferrous sulfate heptahydrate (FeSO₄·7H₂O) was added as a 2.5lb/gal aqueous solution at a rate to provide 75 ppm Fe⁺² on pulp at therepulper of the D1 washer. The pH of the stage was 3.3 and thetemperature was 80° C. H₂O₂ was applied at 0.26% on pulp at the suctionof the stage feed pump.

The fifth or final chlorine dioxide stage (D2) was carried out at atemperature of 80° C., and a pH of 3.9 with 0.16% ClO₂ applied. Theviscosity was 5.0 mPa·s and the brightness was 90.0% ISO.

The iron content was 10.3 ppm, the measured extractives were 0.018%, andthe ash content was 0.1%. Additional results are set forth in Table 1below.

Example 2 (Second Stage Oxidation)

Southern pine chips were cooked in a two vessel continuous digester withLo-Solids® downflow cooking. The white liquor application was 18.7% aseffective alkali (EA) with half being added in the impregnation vesseland half being added in the quench circulation. The quench temperaturewas 165° C. The kappa no. after digesting averaged 14. The brownstockpulp was further delignified in a two stage oxygen delignificationsystem with 2.84% sodium hydroxide (NaOH) and 1.47% oxygen (O₂) applied.The temperature was 92 to 94° C. The Kappa number was 5.6.

The oxygen delignified pulp was bleached in a 5 stage bleach plant. Thefirst chlorine dioxide stage (D0) was carried out with 0.71% chlorinedioxide (ClO₂) applied at a temperature of 63° C. and a pH of 2.5. TheKappa number following then (D₀) stage was 1.7

The second stage was altered to produce a low degree of polymerizationpulp, Ferrous sulfate heptahydrate (FeSO₄·7H₂O) was added as a 2.5lb/gal aqueous solution at a rate to provide 25 ppm Fe⁺², which wasincreased to 40 ppm Fe⁺² on pulp. The pH of the stage was 2.8 and thetemperature was 82° C. H₂O₂ was applied at 0.25% on pulp at the suctionof the stage feed pump.

The third or chlorine dioxide stage (D1) was carried out at atemperature of 79.5° C. and a pH of 2.9, ClO₂ was applied at 0.90% andNaOH at 10.43%. The 0.5% Capillary CED viscosity was between 5.4 and 6.1mPa·s.

The fourth or alkaline extraction stage (EP) was carried out at atemperature of 76° C. NaOH was applied at 1.54%, and hydrogen peroxide(H₂O₂) at 0.28%. The pH was 11.3

The fifth or final chlorine dioxide stage (D2) was carried out at atemperature of 72° C., and a pH of 4.4 with 0.14% ClO₂ applied.

Fiber characteristics are set forth in Table 1, below.

Example 3—WO 2010/138941

Southern pine chips were cooked in a two vessel continuous digester withLo-Solids® downflow cooking. The white liquor application was 8.32% aseffective alkali (EA) in the impregnation vessel and 8.46% in the quenchcirculation. The quench temperature was 162° C. The kappa no. afterdigesting was 27.8. The brownstock pulp was further delignified in a twostage oxygen delignification system with 2.44% NaOH and 1.91% O₂applied. The temperature was 97° C. The first reactor pressure was 779kPa and the second reactor was 386 kPa. The kappa no. after oxygendelignification was 10.3.

The oxygen delignified pulp was bleached in a 5 stage bleach plant. Thefirst chlorine dioxide stage (D0) was carried out with 0.94% ClO₂applied at a temperature of 66° C. and a pH of 2.4.

The second or oxidative alkaline extraction stage (EOP) was carried outat a temperature of 83° C. NaOH was applied at 0.89%, H₂O₂ at 0.33%, andO₂ at 0.20%. The kappa no. after the stage was 2.9.

The third or chlorine dioxide stage (D1) was carried out at atemperature of 77° C. and a pH of 2.9. ClO₂ was applied at 0.76% andNaOH at 0.13%. The 0.5% Capillary CED viscosity was 14.0 mPa·s.

The fourth stage was altered to produce a low degree of polymerizationpulp.

Ferrous sulfate heptahydrate (FeSO₄·7H₂O) was added as a 2.5 lb/galaqueous solution at a rate to provide 150 ppm Fe⁺² on pulp at therepulper of the D1 washer. The pH of the stage was 2.6 and thetemperature was 82° C. H₂O₂ was applied at 1.6% on pulp at the suctionof the stage feed pump.

The fifth or final chlorine dioxide stage (D2) was carried out at atemperature of 85° C., and a pH of 3.35 with 0.13% ClO₂ applied. Theviscosity was 3.6 mPa·s and the brightness was 88.7% ISO.

Each of the bleached pulps produced in the above examples were made intoa pulp board on a Fourdrinier type pulp dryer with an airborne Fläktdryer section. Samples of each pulp were collected and analyzed forchemical composition and fiber properties. The results are shown inTable 1.

TABLE 1 Example Example Example Property units 1 2 3 R10 % 81.5 84.271.6 S10 % 18.5 15.7 28.4 R18 % 85.4 87.6 78.6 S18 % 14.6 12.2 21.4 ΔR3.9 3.6 7.0 Carboxyl meq/100 g 3.14 3.8 3.98 Aldehydes meq/100 g 1.800.74 5.79 Copper No. 1.36 0.69 3.81 Calculated Carbonyl* mmole/100 g2.15 1.03 6.23 CED Viscosity mPa · s 5.0 5.8 3.6 Intrinsic Viscosity [h]dl/g 3.58 4.06 2.52 Calculated DP*** DP_(w) 819 967 511 Glucan % 83.582.9 83.3 Xylan % 7.6 7.4 7.6 Galactan % <0.1 0.3 0.1 Mannan % 6.3 5.86.3 Arabinan % 0.4 0.3 0.2 Calculated Cellulose** % 81.4 81.0 81.2Calculated Hemicellulose % 16.5 15.7 16.3

Example 4—Carboxylating Acid Treatment

Cellulose pulp prepared generally according to Example 1, i.e., fivestage bleaching where oxidation is carried out in the fourth stage, wassubject to a carboxylating acid treatment with chlorous acid in thefifth or D2 acid treatment stage.

The oxidation stage was carried out at a temperature of 80° C. and a pHof 2.85. An iron source was added at a rate to provide 150 ppm Fe⁺².H₂O₂ was applied at 1.5% on pulp at the suction of the stage feed pump.

The carboxylating acid treatment stage (D2) was carried out at atemperature of 80° C. and a pH of 2.89 with 0.6% H₂O₂ and 1.6% NaClO₂applied.

Fiber characteristics are set forth in Table 2, below, comparing thefiber to itself as it moves along the bleaching sequence, specifically,after the D1 stage, after the fourth or oxidation stage and after the D2or acid treatment stage.

TABLE 2 Modified Acid-Treated Bleached Fiber Modified Fiber FiberProperty Unit(s) (After Stage 3) (After Stage 4) (After Stage 5)Viscosity cps 7.68 3.25 3.25 Carboxyl meq/100 g 3.15 3.66 7.28 Aldehydemeq/100 g 0.14 4.73 0.64 Copper No. 0.36 4.15 1.61 Carbonyl meq/100 g0.48 6.8 2.62

Example 5—Carboxylating Acid Treatment

Cellulose pulp prepared generally according to Example 1, i.e., fivestage bleaching where oxidation is carried out in the fourth stage, wassubject to a carboxylating acid treatment with chlorous acid in thefifth or D2 acid treatment stage.

The fourth oxidation stage was carried out at a temperature of 80 and apH of 2.05, An iron source was added at a rate to provide 300 ppm Fe⁺².H₂O₂ was applied at 3.0% on pulp at the suction of the stage feed pump.

The fifth or carboxylating acid treatment stage (D2) was carried out ata temperature of 80° C. and a pH of 2.86 with 0.8% H₂O₂ and 2.4% NaClO₂applied.

Fiber characteristics are set forth in Table 3, below, comparing thefiber to itself as it moves along the bleaching sequence, specifically,after the D1 stage, and after the D2 or acid treatment stage.

TABLE 3 Modified Acid-Treated Bleached Fiber Fiber Property Unit(s)(After Stage 3) (After Stage 5) Viscosity cps 7.68 3.64 Carboxyl meq/100g 3.15 7.77 Aldehyde meq/100 g 0.14 0.36 Copper No. 0.36 1.58 Carbonylmeq/100 g 0.48 2.52

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the disclosure. Accordingly, other embodimentsare within the scope of the following claims.

We claim:
 1. A chemically modified kraft fiber comprising cellulosehaving a carboxyl content greater than about 6 meq/100 g, an aldehydecontent of less than 1 meq/100 g, a copper number of less than about1.6, and a viscosity less than 5.5 mPa·s.
 2. The fiber of claim 1,wherein the carboxyl content is at least about 7 meq/100 g.
 3. The fiberof claim 1, wherein the ISO brightness is from about 85 to about
 92. 4.The fiber of claim 1, having a degree of polymerization of from about350 to about
 1860. 5. The fiber of claim 1, having an aldehyde contentof less than about 0.6 meq/100 g.
 6. The fiber of claim 1, having acarbonyl content ranging from about 1.5 meq/100 g to about 2.5 meq/100g.
 7. The fiber of claim 1, having a viscosity of from about 3 mPa·s toless than 5.5 mPa·s.
 8. The fiber of claim 1, having a viscosity of fromabout 3 mPa·s to about 4 mPa·s.
 9. The fiber of claim 5, having a coppernumber of less than about 1.3.
 10. The fiber of claim 1, having an S10caustic solubility ranging from about 14% to about 30%.
 11. The fiber ofclaim 1, having an S18 caustic solubility less than about 16%.
 12. Thefiber of claim 1, having an R10 value ranging from at least about 72% toabout 82%.
 13. The fiber of claim 1, having an R18 value ranging from atleast about 79% to about 86%.
 14. The fiber of claim 1, wherein thefiber is softwood fiber and having a hemicellulose content of from about12% to about 17%.
 15. The fiber of claim 1, wherein the fiber issoftwood fiber and having a fiber length of at least about 2 mm.
 16. Thefiber of claim 1, wherein the fiber exhibits antimicrobial and/orantibacterial properties.
 17. The fiber of claim 1, wherein the fiber iscontained within an absorbent product that further comprises at leastone super absorbent polymer.